Electrochemical process and device for hydrogen generation and storage

ABSTRACT

Both the reaction of hydride-forming compositions with hydrogen to form hydrides, and the decomposition of such hydrides to release hydrogen may be promoted electrochemically. These reactions may be conducted reversibly, and if performed in a suitable cell, the cell will serve as a hydrogen storage and release device.

This application is a division of co-pending application Ser. No.13/293,432, filed Nov. 10, 2011 and titled, Electrochemical Process andDevice for Hydrogen Generation and Storage.

TECHNICAL FIELD

This invention pertains to processes and devices for storing andgenerating hydrogen using reversible electrochemical reactions to bothpromote decomposition of hydrides for release of hydrogen and formationof hydrides for storage of hydrogen.

BACKGROUND OF THE INVENTION

There are many devices which consume hydrogen and produce power. Forexample, proton exchange membrane fuel cells are being developedcurrently as power sources for a variety of military, transportation,and electronic device applications. Such fuel cells require thathydrogen, sufficient to generate the required power output, be availablewhen required. Meeting this requirement calls for a high density andenergy efficient hydrogen storage technology.

Many such technologies have been proposed and studied including devicesthat store hydrogen: as compressed hydrogen gas; as cryogenic liquidhydrogen; as hydrogen molecules adsorbed on high surface area supports;as hydrogen atoms at low density in metallically bonded solid transitionmetal hydrides; as hydrogen atoms at high density in ionically bondedsolid light metal hydrides; and as hydrogen atoms at high density inpolar covalently bonded solid complex hydrides. Each of these approacheshas limitations.

Those approaches which rely on chemical bonding to store and releasehydrogen are attractive, but input of energy, conventionally as heat, isrequired for hydrogen release. In transition metal hydrides, hydrogenmay be released at moderate temperatures because the hydrogen andtransition metal are relatively weakly bonded with metallic bonds. Buttransition metal atoms have atomic weights of greater than approximately50 atomic mass units and store, at most, approximately two hydrogenatoms per transition metal atom. Thus, the gravimetric storage densityof transition metal hydrides is less than 4 weight percent hydrogen,which is too low for many applications.

Light metal atom hydrides can have high hydrogen densities, up toapproximately 12 weight percent hydrogen. However, the ionic chemicalbonds between the metal and the hydrogen in these hydrides are verystrong and, therefore, high temperatures, beginning at about 280° C. andranging up to 900° C. and greater are needed to release the hydrogen.These temperatures are impractical for many applications.

Hydrogen stored in polar covalently bonded light metal complex hydridescan have storage densities up to 18 weight percent hydrogen. Like lightmetal hydrides, these compounds are generally very strongly bound andtherefore, again, high temperatures are required to release thehydrogen.

There is therefore need for improved methods of chemically storing andreleasing hydrogen.

SUMMARY OF THE INVENTION

This invention provides a device for generating hydrogen gas (H₂) frommetal cation-based hydrogen storage compounds. In some embodiments,after discharge of hydrogen, the device may be regenerated when exposedto hydrogen gas. The device consists of an electrochemical cell, whichcomprises a negative electrode and a positive electrode. Either or bothof these electrodes may contain solid metal cation-based hydrogenstorage compounds in the active electrode material. The electrochemicalcell also contains an electrolyte that ionically conducts the metalcation in the metal cation-based hydrogen storage compound. The devicealso contains external connections for the negative and positiveelectrodes. These connections enable use of an electrical power sourceand circuit to establish an electrochemical potential between thenegative and positive electrodes and enable flow of electrical currentbetween them.

In practices of this invention an imposed electrical potential is usedto decompose solid metal cation-based hydrogen storage compounds in theelectrode material into hydrogen gas. When, for example, the negativeelectrode contains a solid metal cation-based hydrogen storage compound,the flow of electrons away from the negative electrode causes theelectrochemical decomposition of the metal cation-based hydrogen storagecompound into hydrogen gas and metal cations. The hydrogen flows out ofthe electrode and exits the device. It may be delivered, for example, tonearby hydrogen using fuel cell or other hydrogen-consuming device. Themetal cations flow ionically through the electrolyte to the positiveelectrode where they are electrochemically reduced by the electronsflowing into the positive electrode from the circuit. When the positiveelectrode contains a solid metal cation-based hydrogen storage compound,hydrogen is produced at the positive electrode in an analogous manner.In some embodiments hydrogen may be evolved at both the positive andnegative electrodes.

Examples of suitable solid metal cation-based hydrogen storage compoundsfor negative electrode active materials include LiH, LiBH₄ or LiAlH₄.Examples of suitable solid metal cation-based hydrogen storage compoundsfor positive electrode active materials include LiOH and LiNH₂. In manyembodiments of the invention the electrode materials are prepared asfine particles and pressed or bonded to a suitable metallic currentcollector for the respective electrode. Fine particles of electricallyconductive material may also be mixed with the active metal cation-basedhydrogen storage compound to provide for better conduction of electronsto or from the active material in each electrode assembly. The liquidelectrolyte may comprise any solvent and solute combination that ischemically compatible with the negative and positive electrode materialsand that ionically conducts the metal cation in the metal cation-basedhydrogen storage compound(s).

Thus, this invention uses an electrochemical potential to decomposesolid metal cation-based hydrogen storage compounds into hydrogen gas.The use of the electrochemical potential is effective in generatinghydrogen over a wide range of temperatures that may exist in the regionof many hydrogen-consuming devices. Despite fluctuations in such ambienttemperatures it is often unnecessary to heat (or cool) the hydrogengenerator. When desired, it may be preferred to size the electrochemicalcell or groups of cells to deliver a specified volume of hydrogen fordelivery to a hydrogen-consuming device. And in many embodiments, it maybe possible to re-form a metal cation-based hydrogen storage compoundwithout removing the material from the cell by supplying hydrogen to theelectrode and reversing current flow.

A wide range of reactants and reactions may be employed, each with itsindividual capability for storing hydrogen. Preferred hydrides includelow atomic weight, light metal simple hydrides, for example LiH or MgH₂or complex hydrides such as alanates (AlH⁴⁻), amides (NH²⁻) orborohydrides (BH⁴⁻). Such complex hydrides may include light metalcations such as Li⁺, Mg⁺, Na⁺, K⁺ and Ca⁺⁺. Other cations in suchcomplex metal hydrides may undergo similar reactions, but because onefigure of merit of chemical hydrogen storage approaches is thegravimetric efficiency, that is the mass of stored hydrogen per unitmass of compound, heavier cations which reduce the gravimetricefficiency are rarely employed. All preferred compositions will be solidat temperatures of below about 100° C. or so.

Exemplary compositions and reactions, with stored hydrogen percentage byweight indicated in parentheses, include:

2LiH→2Li+H₂ (12.7%)  1.

LiBH₄→B+Li+2H₂ (18.5%)  2.

2LiBH₄+3Al→AlB₂+2LiAl+4H₂ (6.5%)  3.

2LiBH₄+MgH₂→MgB₂+2Li+5H₂ (14.4%)  4.

2LiBH₄+MgH₂+2Al→MgB₂+2LiAl+5H₂ (8.1%)  5.

2Li+2LiOH→2Li₂O+H₂ (3.2%)  7.

2LiH+2LiOH→2Li₂O+2H₂ (6.2%)  8.

4Li+2LiNH₂→2Li+2Li₂NH+H₂ (2.7%)→3Li₃N+2H₂ (5.4%)  9.

2LiH+LiNH₂→LiH+Li₂NH+H₂ (4.3%)→Li₃N+2H₂ (8.6%)  10.

2LiBH₄+LiNH₂→2B+Li₃N+5H₂ (14.9%)  11

2LiBH₄+MgH₂+LiNH₂→MgB₂+Li₃N+6H₂ (12.9%)  12.

2LiH+Mg(NH₂)₂→Li₂Mg(NH)₂+2H₂ (10.5%)  13.

MgH₂→Mg+H₂ (7.6%)  14.

These reactions, and others not listed, may be promoted within anelectrochemical cell, which contains a negative electrode and a positiveelectrode immersed in an electrolyte. Either or both of these electrodesmay be porous and contain solid metal cation-based hydrogen storagecompounds supported on a conductive sheet, mesh or frame fabricated of,for example, Ni, Cu and Al. The electrodes may also contain metallichydride-forming metals and alloys in partially or fully dehydrogenatedand electrically conducting form to convey electrons and facilitate thetransport of hydrogen. These hydride-forming metals and alloys mayinclude Mg, Mg₂Ni, TiFe, ZrNi, ZrMn₂, LaNi₅, and LaNi_(5-x)Sn_(x). Forheightened gravimetric efficiency, only minimal quantities of theelectrically conductive powder, sufficient to ensure at least a currentpath to the hydrogen storage compound may be employed. Some additives,such as LaNi₅, may exhibit some catalytic properties and may thereforebe effective even when present in small quantities. Carbon nanotubes orgraphene may be employed where good electrical conductivity at lowweight fractions is paramount.

A binder material, for example styrene-butadiene rubber orpolyvinyldiene fluoride, may be added, but only to the extent requiredto ensure the integrity and long-term performance of the electrode.Depending on the choice of electrically-conducting hydride-formingmetals and alloys, it may be preferred to incorporate catalyticpromoters (for hydrogen) such as carbon-supported Pd, Pt, or Ni, singly,or in combination as mixtures and alloys.

For reactions involving only a single chemical species, for examplereactions 1., 2., and 14., only a single prepared electrode is requiredand the second electrode may be any (electrically) conductive materialwhich does not react with the electrolyte, for example, lithium, copper,nickel or platinum among many others. For reactions involving more thanone reactive species, for example reactions 3. and 4., another of thereactive species must be incorporated into the second electrode.Generally this will entail fabricating a powder processed electrode asalready described, but a solid conductive electrode may also beappropriate, as in reaction 3. where a solid aluminum metal-basedelectrode may be used.

For reactions involving lithium ion conduction, non-aqueous solutions oflithium salts such as 1 molar LiPF₆ dissolved in a 1:1 mixture ofethylene carbonate and dimethyl carbonate or 1 molar LiClO₄ in dimethylcarbonate may be used as electrolytes.

The cell also includes external connections, attached to the negativeand positive electrodes. These connections enable connection of a powersource and circuit external to the cell to impose an electricalpotential difference between the negative and positive electrodes andenable flow of electrical current between the positive and negativeelectrodes.

During operation to generate hydrogen, the electrical potentialdifference imposed by an external power source causes electrons to flowaway from the negative electrode and into the positive electrode throughthe external circuit.

The device may be reversible so that it may also function to storehydrogen in metal cation-based hydrogen storage compounds. In this case,the electrodes contain initially metal cation-based hydrogen storagecompounds in a dehydrogenated state; the direction of current flow inthe circuit is reversed, promoting flow of electrons from the positiveelectrode to the negative electrode; and hydrogen gas (H₂) is suppliedto either or both of the negative or positive electrodes.

It is preferred that the electrodes be closely spaced, with a spacingranging from about 20 micrometers to about 30 micrometers to minimizeinter-electrode resistance. The electrodes may be planar, co-extensiveand arranged in opposition. Folded configurations in which one electrodeadopts a ‘Z’ or ‘W’ shape with interleaved co-extensive opposingelectrodes in face to face opposition may also be employed as mayspiral-wound electrode arrangements.

In some cell geometries, hydrogen transport over appreciable distancesmay be required to convey hydrogen from its source within the cell tothe cell exterior. Similarly charging hydrogen to the cell interior mayinvolve extensive hydrogen transport. To minimize the extent oftransport by diffusion, it may be preferred to incorporate channels orpassages in the cell extending from the outer electrode and incommunication with the cell exterior. Such channels may enable promptdischarge of hydrogen gas from the electrochemical cell interior duringhydrogen generation and prompt loading of the cell interior withhydrogen during hydrogen storage.

It may be preferred to operate the cell under modest pressure, relativeto atmospheric pressure. Suitably the hydrogen pressure should be atleast about 5 bar or so for compatibility with fuel cell systems whichare commonly operated at elevated pressure.

The pressure of the hydrogen released by the reaction is directlyrelated to the applied potential. Commonly the cited potential requiredto drive the reaction and evolve hydrogen is an equilibrium potentialbased on a hydrogen pressure of one atmosphere. A greater (thanequilibrium) potential will result in a higher pressure of the evolvedhydrogen. Hence, under stable conditions, a suitable hydrogen pressuremay be ‘dialed in’ by appropriately setting the applied potential.

But, because stability of operation may not be assured, it may bepreferred to operate the cell at a potential which, over the expectedrange of any variables will guarantee at least a minimum hydrogenpressure, for example the 5 bar or so appropriate for fuel cells. Apressure regulator, installed in the hydrogen line between the cell andany hydrogen-consuming device, may be used to ensure that thehydrogen-consuming device is isolated from the higher gas pressure.

Alternately, if a hydrogen pressure sensor is used and, optionally, a(hydrogen) flow sensor, the hydrogen pressure and hydrogen demand may beused to control the applied cell potential using either on-off orproportional control

These and other aspects of the invention are described below, whilestill others will be readily apparent to those skilled in the art basedon the descriptions provided in this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a cell for electrochemicalproduction of hydrogen with facing opposed planar electrodes andincorporating hydrogen sensor.

FIG. 2 is a plot comparing the variation in cumulative hydrogenconcentration with time for two electrodes, one containing LiH andanother, control electrode, containing no hydride, while under anapplied bias voltage which was alternated between 3 volts and 0 volts.

FIG. 3 is illustrative of an electrode configuration of a hydrogenstorage and release cell electrode adapted for passage of hydrogen froma cell interior to a cell exterior.

FIG. 4 is illustrative of a configuration of a hydrogen storage andrelease cell adapted for passage of hydrogen from a cell interior to acell exterior.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description of the embodiment(s) is merely exemplary innature and is not intended to limit the invention, its application, oruses.

In vehicle applications, chemical storage and release of hydrogen usingmetal hydrides appears to be an attractive alternative to physicalstorage of hydrogen which may require one or more of, high pressures, orlow temperatures or specially prepared substrates to be viable.

For example, most hydrides, in the absence of a stimulus, are stable atambient and near-ambient temperatures of say 10-100° C. so that they maybe stored without need for low temperature or high pressure. The volumeof hydrogen produced may be changed incrementally by addition of more orless reagent to modify the available vehicle range or to satisfy theneeds of vehicles of different sizes and use patterns. Chemicalreactions for release of hydrogen may be reversed to regenerate andrecharge a spent hydrogen source, either on-vehicle or with suitablepackaging and connection capability, off-vehicle, in conjunction with astorage unit exchange scheme. The extent of chemical reaction may becontrolled to enable a hydrogen flow suitable for the desired vehicleperformance, for example its 0-60 mph acceleration time.

But chemical approaches are also challenged. The more readily decomposedand reconstituted hydrogen storage compounds have only low gravimetricefficiencies while higher gravimetric efficiency hydrogen storagecompounds are more stable and efficiently release and store hydrogenonly at unacceptably high temperatures. These issues generally lessenthe gravimetric and volumetric efficiency of chemical storage. Thelessened efficiency may be evidenced directly through a need for a largemass of low efficiency hydrides, or indirectly, through a need foradditional thermal handling equipment such as insulation, heaters orheat exchangers.

Although heat is the most common form of energy input used to promotechemical reactions, it is known that there is an equivalence between thethermodynamics of a chemical reaction and the electrical potential whichmust be applied to promote the same reaction electrochemically. Hencedecomposition of hydrogen-containing compounds may feasibly be promotedelectrochemically provided the reactant(s), most of which areelectrically non-conductive, may be incorporated into an electricallyconductive electrode and an electrolyte suitable for ionic conductionmay be identified. If these requirements are satisfied it may befeasible to reversibly release and store hydrogen by application of anelectrical potential to two electrodes immersed in a suitableelectrolyte. Advantageously, such electrochemical reactions may beconducted at temperatures of less than 100° C. and possibly even as lowas about 25° C.

Among many others, some potential reactions which may be considered forsuch electrochemically-promoted hydrogen release include:

2LiH→2Li+H₂ (12.7%)  1.

LiBH₄→B+Li+2H₂ (18.5%)  2.

2LiBH₄+3Al→AlB₂+2LiAl+4H₂ (6.5%)  3.

2LiBH₄+MgH₂→MgB₂+2Li+5H₂ (14.4%)  4.

2LiBH₄+MgH₂+2Al→MgB₂+2LiAl+5H₂ (8.1%)  5.

2Li+2LiOH→2Li₂O+H₂ (3.2%)  7.

2LiH+2LiOH→2Li₂O+2H₂ (6.2%)  8.

4Li+2LiNH₂→2Li+2Li₂NH+H₂ (2.7%)→3Li₃N+2H₂ (5.4%)  9.

2LiH+LiNH₂→LiH+Li₂NH+H₂ (4.3%)→Li₃N+2H₂ (8.6%)  10.

2LiBH₄+LiNH₂→2B+Li₃N+5H₂ (14.9%)  11

2LiBH₄+MgH₂+LiNH₂→MgB₂+Li₃N+6H₂ (12.9%)  12.

2LiH+Mg(NH₂)₂→Li₂Mg(NH)₂+2H₂ (10.5%)  13.

MgH₂→Mg+H₂ (7.6%)  14.

where the percentages in parentheses are the percentage by weight ofhydrogen which may be released or stored. The cations in complexhydrides may include, in addition to Li⁺ and Mg⁺⁺ as shown, other lightmetal cations such as Na⁺, K⁺ and Ca⁺⁺. Complex metal hydrides of othercations may exist and may undergo similar reactions, but because theirhigher mass decreases the gravimetric efficiency of the compound,hydrides of such cations are not commonly considered for hydrogenstorage. All preferred compositions will be solid at temperatures ofbelow about 100° C. or so.

The compounds listed include compounds in which the hydrogen is inhydridic form, for example LiH, LiBH₄ or LiAlH₄ and compounds in whichthe hydrogen is in protonic form, including LiOH and LiNH₂.

Hydridic compounds contain hydrogen which is at least partiallynegatively charged with between <0 and −1 elementary units of charge. Insuch hydridic compounds, when employed as the negative electrode of anelectrochemical cell, the flow of electrons away from the negativeelectrode during operation causes the electrochemical decomposition ofthe metal cation-based hydrogen storage compound into hydrogen gas andmetal cations. The hydrogen gas is liberated at the negative electrodeand exits the device. The metal cations are released into theelectrolyte and flow ionically through the electrolyte to the positiveelectrode. At the positive electrode, these metal cations areelectrochemically reduced by the electrons flowing into the positiveelectrode from the circuit.

In protonic compounds, the hydrogen is at least partially positivelycharged with between >0 and +1 elementary units of charge. In anelectrochemical cell, the negative electrode may contain the reduced(metallic) form of the metal cation while the metal cation-basedhydrogen storage compound may be the positive electrode. Duringoperation, the flow of electrons away from the negative electrode causesrelease of the metal in the form of metal cations into the electrolyte(electrochemical oxidation of the metal). The metal cations flowionically through the electrolyte to the positive electrode where theyreact electrochemically with the metal cation-based hydrogen storagecompound and the electrons flowing into the positive electrode from thecircuit. Reaction of the metal cation-based hydrogen storage compoundwith the metal cations and electrons causes decomposition of the metalcation-based hydrogen storage compound into hydrogen gas and a (fully orpartially) dehydrogenated metal cation compound. The hydrogen gas isliberated at the positive electrode.

So, by appropriate choice of compounds and placement of the compounds atthe negative or positive electrode of an electrochemical cell, hydrogenmay be evolved at the negative electrode in an embodiment, at thepositive electrode in another embodiment and at both the negative andthe positive electrodes simultaneously in a yet further embodiment.Reversing the polarity of the impressed voltage and supplying hydrogento the cell has the potential to reconstitute the original hydrogenstorage compositions.

Because many of the suitable compounds are electrically non-conducting,the electrodes must be fabricated with electrically-conductiveconstituents and features to assure that any imposed electricalpotential is effectively applied to the hydride. Generally suchelectrodes are porous and comprise intimate powder mixtures of hydrogenstorage compounds with an electrically conductive medium such as metalor metal alloy, for example Ni, Cu, or Al or their alloys in the form ofpowder or as a foam. Powdered graphite, carbon, carbon nanotubes andgraphene may also be suitable.

The electrodes may also contain metallic, electrically conductinghydride-forming alloys in partially or fully dehydrogenated form toconvey electrons and facilitate the transport of hydrogen. Exemplaryhydride-forming alloys may include Mg, Mg₂Ni, TiFe, ZrNi, ZrMn₂, LaNi₅,and LaNi_(5-x)Sn_(x). Generally the electrically conductive powder ispresent only in quantities suitable to ensure a current path to thehydrogen storage compound. Minimal addition of a binder, for examplestyrene-butadiene rubber or polyvinyldiene fluoride, may be made toensure the integrity of the electrode. Catalytic promoters (forhydrogen), such as carbon-supported Pd, Pt, or Ni, individually or asmixtures or alloys, may also be incorporated into the electrodes.

The electrodes may be fabricated by mechanically milling a mixture ofelectrically conducting and hydride powders to comingle the powders andachieve a desired particle size, typically ranging from 1 micrometer to100 micrometers. It may be preferred to size the resulting powders,suitably ranging from 5 micrometers to 10 micrometers, to assure that,when compacted into an electrode, pores of suitable size and density fortransport of hydrogen are formed. Such pores are intended to facilitateand enhance transport of hydrogen between the electrode interior and theelectrode exterior.

For enhanced electrode cohesion, the particles may be suspended in anextremely dilute (1% to 3%) solution of a binder such asstyrene-butadiene rubber or polyvinylidene fluoride (PVDF) dissolved ina suitable solvent. For many hydrides acetone is a suitable solvent, butacetone is known to react with at least NaAlH₄ leading to hydrogenrelease. To avoid reaction with this and other, similarly-reactinghydrides, alternative solvents such as tetrahydrofuran (THF) or diethylether may be used. After compaction and evaporation of the solvent thebinder will be effective in further securing the particles to oneanother.

The electrode may be supported by, and connected to, an external circuitthrough a conductive support such as a hollow frame, wire mesh or thinfoil. The electrode may have the support incorporated within theelectrode by embedding the electrode into the electrode powder so that,upon compaction, the electrode and the support may be mechanicallyinterlocked. Such an approach is most effective if the support has openfeatures to entrain powder and so is best suited to open frame or wiremesh supports and the like. Alternatively, if the solvent dissolvedbinder approach is used, a powder/binder/solvent slurry or paste may beapplied to the support by spraying painting or by use of a doctor bladeor other similar approach. This may be the preferred approach for morefeatureless supports such as a thin foil but is applicable to allsupport geometries. The approaches may also be combined by compacting a‘near-dry’ powder/binder/solvent mixture around a support andevaporating the remaining solvent so that both the binder and themechanical interlock combine to secure the electrode to its support.Suitable materials for the supports include a metal mesh or a foam,fabricated from, for example Ni, Cu, or Al. Carbon foam may also besuitable.

The electrolyte may be any solvent and solute combination that ischemically compatible with the negative and positive electrodes and thationically conducts the metal cation in the metal cation-based hydrogenstorage compound. A typical non-aqueous electrolyte that is useful forLi⁺ cations is 1 molar LiPF₆ dissolved in 1:1 ethylenecarbonate/dimethyl carbonate. For Mg⁺⁺ ions also, a 1:1 mixture ofethylene carbonate/dimethyl carbonate may serve as a solvent for asuitable magnesium salt.

A suitable device for practice of the invention is shown, schematically,in FIG. 1. The operating features of the device, as illustrated, arethose which result when the reactant and reaction are those of reaction1., that is:

2LiH→2Li+H₂ (12.7%)

It is worth noting that, if thermally driven, a temperature of ˜940° C.would be required to produce hydrogen at a pressure of about 1 bar dueto the high positive Gibbs free energy of the reaction (ΔG=137kJ/mol-H₂), large entropy change (ΔS=148 J/K-mol-H₂), and high enthalpyof reaction (ΔH=181 kJ/mol-H₂) but that an electrochemically drivenreaction may be achieved by application of only 0.71 volts.

In this reaction, the metal cation-based hydrogen storage compoundcontaining hydrogen in hydridic form in the negative electrode is LiH,the metal cation is Li⁺, and hydrogen is released from the negativeelectrode. The overall reaction results from two half cell reactions,

2LiH→2Li⁺+H₂+2e ⁻  1a. at the negative electrode, and

2Li⁺+2e⁻→2Li  1b at the positive electrode

Referring to FIG. 1, an electrochemical cell 10 comprises an electrolyte12, and electrodes 16,18 contained within a container 14, where theelectrodes are connected to an external circuit through leadwires 20,22. Cell 10 may further include a hydrogen sensor 24. The circuit may becompleted by attaching electrode leadwires 20, 22 to power source 30. Itwill be appreciated that this representation is schematic only and thateach of the cell elements, including, but not limited to, container 14,electrodes 16,18 and electrolyte 12 may, in a particular implementation,adopt different shapes and configurations.

Electrode 16 may comprise LiH and electrode 18 may comprise lithiummetal. Upon connection of the electrode leadwires 20,22 of cell 10 topower source 30, half cell reaction 1a, will occur at electrode 16causing an electronic current to flow in direction of arrow 28, an ioniccurrent of Li⁺ ions to migrate through electrolyte 12 to electrode 18 inthe direction of arrow 26, and evolution of hydrogen gas 32. Lithiumions, on reaching electrode 18, may then be reduced to lithium metal asshown at reaction 1b. Electrode 18 since it does not chemicallyparticipate in the reduction of lithium ions to metal may be fabricatedof any convenient electrical conductor which does not react with theelectrolyte or alloy with lithium metal.

Further understanding of the invention may result from consideration ofthe following example, which similarly involves the evolution of H₂ byelectrochemical processing of LiH by the overall process of reaction 1.and the half cell reactions of reactions 1a. and 1b.:

A negative electrode was fabricated by mechanically milling a mixture of10 wt % (0.12 grams) LiH with 90 wt % (1.08 grams) LaNi₅ in a Fritsch P6planetary mill for 1 hour at 400 rpm. Such a procedure may be expectedto result in particle agglomerates ranging from 1 to 10 micrometers insize, each agglomerate comprising smaller individual crystallites. TheLaNi₅ was used to provide electrical conductivity, mechanical stability,and to facilitate hydrogen transport. The milled mixture was formed intoa disk electrode approximately 10 millimeters in diameter by pressing itonto a Ni grid using a screw press and applying a torque of about 300inch-lbs for about 10 seconds. A typical electrode, including itssupport mesh, weighed about 0.1 grams. The negative electrode was usedin a laboratory electrochemical cell such as is shown in FIG. 1 with aLi foil positive electrode and an electrolyte consisting of 1 molarLiPF₆ dissolved in a 1:1 mixture of ethylene carbonate and dimethylcarbonate maintained under an argon atmosphere. External connections(not shown in FIG. 1) in this laboratory cell were made with alligatorclips. An electrical potential of 3 volts, was applied by a Solartron1287 potentiostat. This corresponds to an overpotential relative to theequilibrium potential of 0.71 volts and was selected to drive thereaction. Release of hydrogen occurred according to Reaction 1. listedabove and was monitored using a Model 700 hydrogen sensor from H₂Scan.

Some results obtained at about ambient temperature with the set-up andreactants of FIG. 1 are shown in FIG. 2 for two negative electrodecompositions. One electrode composition containing LiH was fabricated bythe process described above. In a separate test another electrode,omitting LiH from the composition, was processed similarly. The resultsfrom the LiH-containing electrode are shown by curve 40 and the resultsfrom the electrode without LiH are shown at curve 50. In both cases, thehydrogen concentration in the open volume of container 14, measured byhydrogen sensor 24 (FIG. 1), is shown as a function of time.

FIG. 2 also illustrates the effects of applying and not applying the 3volt electric potential. The potential is applied, for theLiH-containing electrode over the time window from about 4500 to 6500seconds encompassed by box 42; for the electrode which was preparedwithout LiH, the potential was applied over the time period extendingfrom about 1500 seconds to about 5200 seconds and encompassed by(dashed) box 52.

In both cases, at early times, some modest evolution of hydrogen wasindicated even in the absence of an applied potential. It is believedthat this is attributable to a combination of instrumental drift overthe long time-frame of the test and, in the case of the LiH-containingelectrode, reflects some oxidation, possibly with some H₂O as animpurity in the electrolyte. Importantly, in the case of the control, ordummy electrode, the rate of apparent evolution of hydrogen wasunaffected by application of a potential. The portion of curve 50encompassed by box 52 is the time during which a potential of 3 voltswas applied: clearly the data is represented by one smooth andcontinuous curve independent of whether or not a potential is applied.

However, as shown by curve 40, representative of an electrode containingLiH, a dramatic increase in hydrogen evolution occurred when a potentialof 3 volts was applied, that is in the portion of the curve encompassedby box 42. If the applied voltage had no effect, the hydrogen generationwould have continued to follow a simple extrapolation of the earlyportion of the curve as shown by curve 40′. On the basis of suchextrapolation, after test termination or about 8700 seconds or so, ahydrogen concentration of about 0.13% or so would have been recorded,whereas a much larger hydrogen concentration of about 0.41% or so wasactually recorded. When the potential was increased, corresponding toedge 44 of box 42, the rate of change of hydrogen concentration withtime began to increase and shortly reached a limiting value more than 10times greater than the initial rate. When the electrical potential wasremoved, corresponding to edge 46 of box 42, the observed rate ofhydrogen evolution asymptotically declined to about its initial rate.

These tests confirm that provided a hydrogen-containing compound ispresent at an electrode that it may be reacted electrochemically to giveup hydrogen. Since the reactions are reversible, applying an opposingpotential with the cell under a hydrogen atmosphere may enablereversible storage and release of hydrogen at temperatures far lowerthan are required for thermally driven reactions. It will be appreciatedthat reversal of some reactions may occur more readily or morecompletely than others, depending for example on the number, scale anddistribution of reactants.

As noted, other compounds and reactions may also be promotedelectrochemically. Further details of these previously-identifiedreactions 2.-14. are provided below. Each overall reaction is followed,in parentheses, by its gravimetric efficiency for hydrogen storage (in%) and the equilibrium electrochemical potential (in volts, V)

LiBH₄→B+Li+2H₂ (18.5%) (E=0.33 V)  2.

In this reaction, the metal cation-based hydrogen storage compoundcontaining hydrogen in hydridic form in the negative electrode is LiBH₄,the metal cation is Li⁺, and hydrogen is released from the negativeelectrode.

The half cell reactions are:

LiBH₄→Li⁺+B+2H₂+2e ⁻ at the negative electrode, and

Li⁺+2e ⁻→Li at the positive electrode.

2LiBH₄+3Al→AlB₂+2LiAl+4H₂ (6.5%) (E=0.01 V)  3.

In this reaction, the metal cation-based hydrogen storage compoundcontaining hydrogen in hydridic form in the negative electrode is LiBH₄,the metal cation is Li⁺, and hydrogen is released from the negativeelectrode. Aluminum is included in both the negative and positiveelectrodes to destabilize the LiBH₄ by stabilizing the dehydrogenationproducts B (in the negative electrode as AlB₂) and Li (in the positiveelectrode as LiAl).

The half-cell reactions are:

2LiBH₄+Al→2Li⁺+AlB₂+4H₂+2e ⁻ at the negative electrode, and

2Li⁺+2Al+2e ⁻→2LiAl at the positive electrode.

2LiBH₄+MgH₂→MgB₂+2Li+5H₂ (14.4%) (E=0.21 V)  4.

In this reaction, the metal cation-based hydrogen storage compoundcontaining hydrogen in hydridic form in the negative electrode is LiBH₄,the metal cation is Li⁺, and hydrogen is released from the negativeelectrode. The LiBH₄ is destabilized by the inclusion of MgH₂ in thenegative electrode.

The half-cell reactions are:

2LiBH₄+MgH₂→2Li⁺+MgB₂+5H₂+2e ⁻ at the negative electrode, and

2Li⁺+2e ⁻→2Li at the positive electrode.

2LiBH₄+MgH₂+2Al→MgB₂+2LiAl+5H₂ (8.1%) (E=0.11 V)  5.

In this reaction, the metal cation-based hydrogen storage compoundcontaining hydrogen in hydridic form in the negative electrode is LiBH₄,the metal cation is Li⁺, and hydrogen is released from the negativeelectrode. The LiBH₄ is destabilized by the inclusion of MgH₂ in thenegative electrode and Al in the positive electrode.

Here the half-cell reactions are:

2LiBH₄+MgH₂→2Li⁺+MgB₂+5H₂+2e ⁻ at the negative electrode, and

2Li⁺+2Al+2e ⁻→2LiAl at the positive electrode

2Li+2LiOH→2Li₂O+H₂ (3.2%) (E=−1.23 V)  7.

In this reaction, the metal cation-based hydrogen storage compoundcontaining hydrogen in protonic form in the positive electrode is LiOH,the metal cation is Li⁺, and hydrogen is released from the positiveelectrode. In this case—note the change in sign of the appliedelectrical potential—the reaction is exothermic, and thus, ideally, noapplied potential would be necessary to release the hydrogen. However,in practice some applied potential could still be necessary to overcomethe overpotential and achieve desired rates.

The half-cell reactions are:

2Li→2Li⁺+2e ⁻ at the negative electrode, and

2Li⁺+2LiOH+2e ⁻→2Li₂O+H₂ at the positive electrode.

2LiH+2LiOH→2Li₂O+2H₂ (6.2%) (E=−0.26 V)  8.

In this reaction, the metal cation-based hydrogen storage compound isLiH in the negative electrode and LiOH in the positive electrode, themetal cation is Li⁺, and hydrogen is released from both electrodes.

Here the half-cell reactions are:

2LiH→2Li⁺+H₂+2e ⁻ at the negative electrode, and

2Li⁺+2LiOH+2e ⁻→2Li₂O+H₂ at the positive electrode

4Li+2LiNH₂→2Li+2Li₂NH+H₂ (2.7%)→3Li₃N+2H₂ (5.4%)  9.

In this reaction, the metal cation-based hydrogen storage compoundcontaining hydrogen in protonic form in the positive electrode is LiNH₂,the metal cation is Li⁺, and hydrogen is released from the positiveelectrode in two reaction steps.

2LiH+LiNH₂→LiH+Li₂NH+H₂ (4.3%)→Li₃N+2H₂ (8.6%)  10.

In this reaction, the metal cation-based hydrogen storage compound isLiH in the negative electrode and LiNH₂ in the positive electrode, themetal cation is Li⁺, and hydrogen is released from both electrodes intwo steps.

2LiBH₄+LiNH₂→2B+Li₃N+5H₂ (14.9%)  11.

In this reaction, the metal cation-based hydrogen storage compound isLiBH₄ in the negative electrode and LiNH₂ in the positive electrode, themetal cation is Li⁺, and hydrogen is released from both electrodes.

2LiBH₄+MgH₂+LiNH₂→MgB₂+Li₃N+6H₂ (12.9%)  12.

In this reaction, the metal cation-based hydrogen storage compound isLiBH₄ in the negative electrode and LiNH₂ in the positive electrode, themetal cation is Li⁺, and hydrogen is released from both electrodes. TheLiBH₄ in the negative electrode is destabilized by inclusion of MgH₂.

2LiH+Mg(NH₂)₂→Li₂Mg(NH)₂+2H₂ (10.5%)  13.

In this reaction, the metal cation-based hydrogen storage compound isLiH in the negative electrode and Mg(NH₂)₂ in the positive electrode,the metal cation is Li⁺, and hydrogen is released from both electrodes.

MgH₂→Mg+H₂ (7.6%) (E=0.19 V)  14.

In this reaction, the metal cation-based hydrogen storage compound isMgH₂ in the negative electrode, the metal cation is Mg²⁺, and hydrogenis released from the negative electrode.

The above listing of reactions is exemplary and not limiting.Alternative suitable hydrides include low atomic weight, light metalsimple hydrides, for example LiH or MgH₂ or complex hydrides such asalanates (AlH⁴⁻), amides (NH²⁻) or borohydrides (BH⁴⁻). Such complexhydrides may include light metal cations such as Li⁺, Mg⁺, Na⁺, K⁺ andCa⁺⁺.

It will be appreciated that the cell configuration shown in FIG. 1 isillustrative of the elements of a cell but that alternativeconfigurations and arrangements of the cell elements may be employed.

For example, for cell efficiency it is preferred that the electrodes beclosely spaced, say 20-30 micrometers apart with a porous, electricallynon-conducting separator between them. Such a separator may allowpassage of ions from one electrode to another and permitting migrationof hydrogen from the electrodes while preventing electrode to electrodecontact and short-circuiting. The output of a cell will scale with themass of hydride and hence with the electrode area. So, for a high outputcell, the electrodes should be extensive.

It is preferred to utilize the entire area of the electrode, but theelectrical potential difference which must be applied to promotereaction depends on the pressure of the hydrogen generated. Thushydrogen, if rapidly generated on an electrode at a location where thehydrogen lacks easy passage to the electrode edge where it may becollected and/or removed, may steadily increase in pressure, reducingthe rate of reaction. In an extreme case where the hydrogen is largelytrapped, the increased pressure may substantially choke off the reactionentirely.

It is apparent that if the sole means for removing hydrogen is at theedges of the electrodes that much of the electrode area will be ill-usedunless the electrodes are small, say from about 100 square millimeter orabout 10 millimeters by 10 millimeters or so to about 1000 squaremillimeters. If electrodes of larger area are to be used, they may beadapted or packaged to lessen the distance over which hydrogen must betransported. Typically this will result in more compact electrodeconfiguration, for example spiral wound cells or Z-fold or W-foldgeometries with interleaved electrodes of the opposite sign and oflimited extent. For these configurations it may be preferred to form theelectrode as a thin deposit on a foil, optionally perforated forenhanced mechanical interference between the electrode material and thesupport for enhanced electrode retention.

It may also be appropriate to make provision to more readily transportthe hydrogen generated at the center of the electrode away from theelectrode. A possible configuration is shown at FIG. 3 in which a cellis to be assembled by inserting a grouping of electrodes 64 with aporous spacer 62 on either side into the folds 70 of a folded secondelectrode 60 by advancing in the direction of arrows 68. For clarityonly one such grouping is shown. Each of electrodes 60 and 64 as well asspacers 62 has substantially equally-sized openings 66, which, when theelectrodes and spacer are assembled together align to form a continuouschannel for hydrogen collection through the folded cell. Connectionmeans for attachment of the electrodes to an external power source 73are illustrated as tabs 63 and 71.

Another embodiment, a representative example of which is shown in FIG.4, uses similar interleaved electrode elements 60′ and 64′ and spacers62′ inserted into folds 70. But in this embodiment a continuous channel72 for hydrogen collection is interposed between the electrodes. Channel72 is a tube, here shown as being of circular cross-section, for lateraltransport of hydrogen and has openings 74 in the wall 76 to permitingress of hydrogen from the positive or negative electrode of the cell.Offsetting the channels 72 as shown, enables a more compactconfiguration and the configuration may be rendered yet more compact byusing an oval tube oriented with its long axis generally parallel to theactive faces of the electrodes. As in the previous embodiment,connection means for attachment of the electrodes to an external powersource 73 are shown, here tabs 63′ and 71′.

The merits of the cell configurations shown in FIGS. 3 and 4 have beendescribed in the context of hydrogen release. But either of theembodiments shown will offer similar benefits in supplying hydrogen tothe cell interior to make the best use of the total available electrodearea if the cell is being regenerated.

It may be preferred to operate the cell under modest pressure, relativeto atmospheric pressure. Suitably the hydrogen pressure should be atleast about 5 bar or so for compatibility with fuel cell systems whichare commonly operated at elevated pressure, but, for control purposes,as discussed further below, it may be advantageous to operate the cellat pressures of up to 20 bar or so.

In all of the hydrogen release reactions 1.-14., increase in thehydrogen pressure will, by LeChatelier's principle, favor the reactantsrather than the reaction products and so limit hydrogen evolution. Butthis may be overcome by increasing the cell potential. Under stableconditions, the pressure of the evolved hydrogen gas may be controlledby controlling the applied electrical potential. This may be used tocontrol the pressure so that the process may be run essentially openloop relying on only the applied potential to ensure that hydrogen isevolved at a suitable pressure.

In practice, kinetic and environmental considerations, like the level ofexhaustion of the cell and the cell temperature, may result in a morevariable hydrogen pressure under constant electrical potential andrequire a more sophisticated control algorithm. This could entailinstalling a hydrogen pressure sensor and, optionally, a (hydrogen) flowsensor, in the cell and using the hydrogen pressure and hydrogen demand,estimated from the hydrogen flow rate, to control the applied cellpotential using either on-off or proportional control

A simpler approach is to operate the cell open loop but at a potentialwhich, over the expected range of any variables, will guarantee at leasta minimum hydrogen pressure, for example, the 5 bar or so appropriatefor fuel cells or other intended use. When conditions exceed theminimum, hydrogen will be generated at a higher pressure, but a pressureregulator, installed in the hydrogen line between the cell and anyhydrogen-consuming device, may be used to ensure that thehydrogen-consuming device is isolated from any higher hydrogen gaspressure. It may be preferred to install a small storage tank betweenthe cell and the regulator to buffer any surges in output or short-livedperiods of high demand.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements aswould be apparent to those skilled in the art. Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A method of reversibly storing hydrogen, the method comprisingapplying a suitable electrical potential difference between first andsecond spaced apart electrodes in facing relation with at least one ofthe electrodes comprising a hydride-forming composition, aborohydride-forming composition, an alanate-forming composition, anamide-forming composition, an imide-forming composition, or ahydroxide-forming composition, each of the first and second electrodesbeing in contact with a non-aqueous electrolyte suited for ionicconduction, the electrodes and electrolyte being under a hydrogenatmosphere.
 2. The method of reversibly storing hydrogen recited inclaim 1 in which at least one electrode comprises one or more of Ni, Cu,Al, Mg, Ni, Ti, Fe, Zr, Mn, La, Sn, graphite and carbon.
 3. The methodof reversibly storing hydrogen recited in claim 1 in which at least oneelectrode comprises one or more of carbon-supported platinum, palladium,and nickel.
 4. The method of reversibly storing hydrogen recited inclaim 1 in which the hydride-forming composition, borohydride-formingcomposition, alanate-forming composition, amide-forming composition,imide-forming composition, or hydroxide-forming composition comprisesone or more of Li, Mg, Na, K, and Ca.
 5. A cell for release and storageof hydrogen, the cell comprising two spaced apart, facing electrodesadapted for connection to an external source of electricity suitable forestablishing an electrical potential difference between the electrodes,the electrodes being held apart by a porous, electricallynon-conducting, spacer, each electrode being in contact with a fluid,non-aqueous ion-conducting electrolyte suitable for transport of aselected ionic species, and at least one of the electrodes comprising ahydride, a borohydride, an alanate, an amide, an imide, or a hydroxide.6. The cell for release and storage of hydrogen recited in claim 5 inwhich at least one of the electrodes comprises one or more of LiH,LiBH₄, MgH₂, LiOH and LiNH₂.
 7. The cell for release and storage ofhydrogen recited in claim 5 in which the electrode area is between about100 square millimeters to about 1000 square millimeters.
 8. The cell forrelease and storage of hydrogen recited in claim 5, the cell furthercomprising features for passage of hydrogen between the cell exteriorand the cell interior.
 9. The cell for release and storage of hydrogenrecited in claim 8, in which the hydrogen-passing features comprisealigned openings in the electrodes.
 10. The cell for release and storageof hydrogen recited in claim 8, in which the hydrogen-passing featurescomprise tubular channels extending along one dimension of theelectrodes.
 11. A method of reversibly storing hydrogen, the methodcomprising: applying a potential difference between first and secondspaced-apart electrodes, immersed in a non-aqueous electrolyte, and infacing relation in an electrochemical cell, in which the first electrodecomprises one or more of the metal elements Li, Mg, Na, K and Ca and thesecond electrode comprises a composition capable of storing ahydrogen-containing composition formed by reaction with hydrogen andcations of the one or more of the metal elements in the presence of thenon-aqueous electrolyte, and concurrently contacting the secondelectrode with hydrogen gas, the applied electrical potential causingthe electrochemical transport of metal cations of the one or more metalelements from the first electrode through the non-aqueous electrolyte tothe second electrode where the metal cations react, under the appliedpotential, with hydrogen gas to store hydrogen as a hydrogen-containingcomposition comprising the one or more metals and hydrogen in the secondelectrode, the thus-formed hydrogen-containing composition being capableof releasing hydrogen gas, and cations of the one or more metals, whenan electrical potential of opposite polarity is applied between thefirst and second electrodes.
 12. A method of reversibly storing hydrogenas recited in claim 11, in which the second electrode further comprisesparticles of one or more of Ni, Cu, Al, Mg, Ni, Ti, Fe, Zr, Mn, La, Sn,graphite, and carbon in an amount to provide electrical conductivitybetween and to the hydrogen-containing composition of the electrode. 13.A method of reversibly storing hydrogen as recited in claim 11, in whichat least one electrode further comprises particles containing one ormore of platinum and palladium as a catalyst for the reaction ofhydrogen with the metal cations and the hydrogen-containing composition.14. A method of reversibly storing hydrogen as recited in claim 11 inwhich lithium ions are transported from the first electrode through thenon-aqueous electrolyte to the second electrode to react with hydrogengas and to form a lithium-containing hydride for the reversible storageof hydrogen.
 15. A method of reversibly storing hydrogen as recited inclaim 11 in which the hydrogen-containing composition is one or more ofa hydride, a borohydride, an alanate, an amide, an imide, or ahydroxide.
 16. A method of reversibly storing hydrogen, the methodcomprising: applying a potential difference between first and secondspaced-apart electrodes, immersed in a non-aqueous electrolyte, and infacing relation in an electrochemical cell in which the first electrodecomprises lithium and the second electrode comprises a compositioncapable of forming and/or storing a hydrogen-containing composition uponreaction of hydrogen and lithium cations in the presence of thenon-aqueous electrolyte, and concurrently contacting the secondelectrode with hydrogen gas, the applied electrical potential causingthe electrochemical transport of lithium cations from the firstelectrode through the non-aqueous electrolyte to the second electrodewhere the lithium cations react, under the applied potential, withhydrogen gas to store hydrogen as a hydrogen-containing compositioncomprising lithium and hydrogen in the second electrode, the thus-formedhydrogen-containing composition being capable of releasing hydrogen gas,and lithium cations, when an electrical potential of opposite polarityis applied between the first and second electrodes.
 17. A method ofreversibly storing hydrogen as recited in claim 16, in which the secondelectrode further comprises particles of one or more of Ni, Cu, Al, Mg,Ni, Ti, Fe, Zr, Mn, La, Sn, graphite, and carbon in an amount to provideelectrical conductivity between and to hydrogen-containing compositionof the electrode.
 18. A method of reversibly storing hydrogen as recitedin claim 16, in which at least one electrode further comprises particlescontaining one or more of platinum and palladium as a catalyst for thereaction of hydrogen with the lithium cations and thehydrogen-containing composition.
 19. A method of reversibly storinghydrogen as recited in claim 16 in which the hydrogen-containingcomposition is one or more of a hydride, a borohydride, an alanate, anamide, an imide, or a hydroxide.
 20. A method of reversibly storinghydrogen as recited in claim 11 in which the second electrode comprisescompacted particles of an electrically conductive material.